Prior art key telephone systems have been developed whereby a particular telephone station set has the capability of direct access or connection to a plurality of lines or trunks connected to a central office. Key telephone systems have also been developed in which key telephone station sets may signal and communicate with other key telephone station sets in the system in a so-called "intercom" mode. In older systems these features are realized by terminating all of the line conductors for each of these lines, whether they be central office lines or intercom lines, directly at the station set, and providing a key or button for each line. The particular line to be used by the telephone station set is user accessed by depressing the key for that line. The depressing of the key physically creates an electrical connection between the station set and the conductors to that line. Additional keys have been provided for placing a line in a hold condition where a second line is to be connected to the station set in order to complete a call over the second line without disrupting the connection to the first line. Many other special services have also been provided in the past such as conference calling, exclusion and exclusion release, paging, etc.
As mentioned above, in the prior art key telephone systems, each line has been physically terminated at the station set with its plurality of conductors so that as many as fifty conductors have had to be cabled to a station set in order to control as few as six days on a set. With modern key telephone systems increasing the number of lines and system features which may be conrolled at each station set, the number of conductors which must be cabled to each station set has become intolerably large.
In an attempt to reduce the number of these conductors, systems have been developed, and are known, where only a few conductors are cabled from a central control unit to each station set, whereby all of the previously known features are realized through the technique of time division multiplex signalling over a few conductors between the key service unit and the telephone station set. Such systems frequently employ a central computer which may be a central control unit or a key service unit (KSU) which may be a data processor or digital computer in order to communicate with each of the station sets connected to the system. Examples of these prior art systems which have used computerized key service units to control the key telephone systems are many.
For example, U.S. Pat. No. 3,691,310 describes an electronic key telephone system in which each telephone station set, regardless of the number of keys with which it may be equipped, may be connected to a local switching network via only a single pair of tip and ring conductors and a data channel. The data channel comprises four conductors over which time division multiplex signals are transmitted between each telephone station set and a key service unit. The prior art paper entitled, "The Modular Electronic Key Telephone System" by Knollman, Reynolds and Simon, published in the International Conference of IEEE (paper number 770-CP-254) describes a system in which key telephone station sets are connected to a key service unit by six conductors, two conductors of which are a tip and ring pair serving as a voice path. Two conductors serve as a digital transmission path from the key service unit (KSU) to the key telephone set, the final two conductors serving as a digital transmission path from the key telephone set to the key service unit. The Knollman, et al. paper describes a key telephone set having six keys or buttons in which a three bit code is assigned for each button. One code corresponds to the switch hook position, with other codes corresponding to the six keys. Each key is assigned a three bit code by which a data word is provided between the key service unit and the telephone station set. It is apparent that this procedure is acceptable for a key telephone station set with a small number of keys because the data word is of a manageable size. However, as service increases for a key telephone set where a large number of central office lines and features is desired, assigning a digital code for each button becomes unmanageable.
Therefore one object of this invention is to provide a key telephone set having a built-in processing means by which each key on the key telephone set may be interrogated and its status transmitted to the key service unit and whereby the processing means in the telephone set received messages from the key service unit regarding the lighting of visual indicators and the control over the telephone set.
It is a more specific object of this invention to provide a key telephone set having a stored program digital computer within it, whereby a stored program generates and receives the input and output signals between the telephone set and the key service unit and controls the electrical and physical status of the key telephone set.
An advantage of this invention in which the key telephone station set has a stored program computer built into it is that it may operate in a total key telephone system so that the following features may be provided: central office line selection; placing an accessed central office line on hold; privacy on a central office line; privacy release, the capability of having either a built in speakerphone or the hybrid network of the handset simultaneously connected via two independent audio frequency communication paths to the key service unit; means for the user of the telephone set to turn the built-in speakerphone on or off, dial selective intercom calling with privacy, the provision for answering an intercom call "hands free" or via the handset; and other features such as "hotline", "paging," "do not disturb," "flash," line identification, central office line conferencing, etc.